1. Field of the Invention
The present invention relates to a technology for detecting an area specified based on a mark of a specific color out of three primary colors from a printed image.
2. Description of the Related Art
Recently, an electronic watermark technique of embedding invisible data into image data has been actively developed. For example, Japanese Patent Application Laid-Open No. 2004-349879 describes a technique of dividing image data into plural blocks, and embedding plural codes into the blocks by relating one code to each block based on a magnitude relation of characteristics of the block such as average concentration of the block.
A technique of embedding data into printed or displayed image and using the image is also being developed. For example, when a uniform resource locator (URL) is embedded into a printed image, a user can fetch the printed image with an imaging device installed in a portable telephone to extract the URL, thereby automatically accessing a web page by using the extracted URL.
To fetch data from the image, it is necessary to specify an area of the image where the data is embedded. For specifying an image area, a technique of embedding a square mark into the printed image to specify a data-embedded area is developed (see, for example, Japanese Patent Application Laid-Open No. H7-254037).
Japanese Patent Application Laid-Open No. H9-018707 discloses a technique of embedding a yellow mark into a printed image to make the embedded mark not easily noticeable. Japanese Patent Application No. 2004-110116 describes a mark that is designed not to be affected by variation in the size of an image picked up with an imaging device.
However, when a background color of the mark is not constant, and when a character or a line in another color is present around the mark or on the mark, detection of the mark becomes difficult. To facilitate the detection of the mark, a technique of separating a specific color area from the image area is developed (see, for example, Japanese Patent Application Laid-Open No. 2002-216119).
However, the conventional techniques require an enormous calculation for color space conversion for each pixel when separating a specific color area.